Anaerobic compositions contain a (meth)acrylic ester monomer as the main component and have the property of stably retaining a liquid state over long, without gelling, when in contact with air or oxygen and rapidly curing upon isolation from or exclusion of the air or oxygen. Owing to this property, these compositions are used for the bonding or fixing of screws, bolts, etc., securing of mating parts, bonding of flanges to each other, sealing, filling of void spaces formed in cast parts, and other applications.
Recently, there is a desire for the development of an anaerobic composition having an extremely high polymerization rate so as to cope with speedups in production lines for producing industrial products. A conventional technique for increasing the polymerization rate of an anaerobically curable composition is to incorporate a polymerization accelerator, e.g., a sulfimide, mercaptan, or amine, and a polymerization initiator into resin ingredients.
However, use of such conventional polymerization accelerators impairs the storage stability of the anaerobically curable composition, and it is extremely difficult to regulate the addition amount of a polymerization initiator so as not to reduce the storage stability. No anaerobically curable composition has been obtained so far which is sufficiently applicable to production lines, etc.
An anaerobically curable composition basically contains two coexistent ingredients, i.e., an ingredient serving to polymerize a radical-polymerizable monomer and an ingredient serving to inhibit the polymerization of the radical-polymerizable monomer, and can be made to perform its function by suitably balancing the two ingredients with each other. Virtually, however, various factors should be supposed in determining the degree of storage stability, because there may be cases where the composition, when sold as a product, is exposed to a high temperature in a vehicle during transportation or directly irradiated with sunlight during storage. Namely, sufficient storage stability should be imparted even at the sacrifice of curability and it has been impossible to enhance polymerization curability to an extremely high degree.
Furthermore, the conventional anaerobically curable compositions cannot show a sufficient curing performance only with the condition that they are isolated from air or oxygen. Namely, in order for the conventional anaerobically curable compositions to exhibit a sufficient bonding strength, they should be applied to metallic adherends. This is because metal atoms, metal molecules, metal ions, or the like contained in the adherends function as a polymerization catalyst. There has hence been a problem that when the conventional anaerobically curable compositions are applied to plastics or inert metals such as deposit surfaces of plated articles, the compositions suffer a curing failure or cure to only have an insufficient bonding strength.
The present invention is to solve the above-described problems in the conventional techniques. Accordingly, an object of the invention is to provide an anaerobically curable composition which has improved adhesion properties without impairing storage stability.
Another object of the invention is to provide an anaerobically curable composition which is useful for application to inert metals and plastics, which have been difficult to bond with any conventional anaerobically curable composition.